Permanent Magnet, PM, electrical machines provide robust and compact motors and generators in many industries, such as aerospace for powers up to several 10's of kW's and marine propulsion systems and wind turbine plants up to and potentially above 5 MW.
The fixed magnets located on the rotor of a PM machine result in a rotating magnetic field having a constant magnitude. This means that the magnitude and frequency of the generated output voltage varies with the rotor speed. In aerospace applications, the rotor speed will be generally controlled by and proportional to the speed of the gas turbine jet engine and the drive shaft taken from one of the spools. In current applications, this speed may vary during a flight cycle by as much as 10:1 which relates to significant changes in voltage and frequency beyond acceptable limits for many applications. Consequently, the variable voltage and frequency is conditioned by a fully rated AC to DC converter which is arranged to produce a constant DC voltage by controlling the reactive power flow in the machine stator windings.
In marine propulsion systems shaft generators driven by the main propulsion engine(s) to provide electrical power are limited by the fact that the ship's electrical system normally requires a fixed frequency. This means that engine speed has to be kept constant which is generally inefficient. The use of a power electronics converter in a marine electrical network has the benefit that the main engine can operate at an optimum speed to maximise its efficiency, effectively decoupling it from the requirement to operate at fixed speed to support the fixed frequency marine electrical network.
FIG. 1 shows a typical drive arrangement 110 for providing a DC voltage from a PM machine. Thus, there is depicted a machine rotor 112 which carries a permanent magnet 114 and which is driveably connected to a prime mover, for example a gas turbine engine via an appropriate transmission (not shown). The rotor 112 is rotatably located within the stator 116 of the machine such that a voltage is induced in the stator windings upon rotation of the permanent magnet. The stator is electrically connected to and outputs current to an active converter 118 which is operated to provide a constant DC voltage 120 for a load (not shown).
In many applications, the PM machine is only required to act as an electrical power generator in which case the fully active, bidirectional converter as shown in FIG. 1 is unnecessary and a simpler unidirectional conditioning scheme can be used. FIG. 2 shows an electrical system 210 which includes a PM machine 212 which outputs a current to a 6-pulse diode rectifier 214 and single transistor DC to DC converter 216. Here the 6-pulse diode rectifier 214 converts the variable magnitude, variable frequency voltage provided by the PM machine 212 to a variable magnitude direct voltage. This is conditioned by the DC to DC converter 216 to provide a constant direct voltage to a direct voltage 218 power bus. It will be appreciated that in some applications, the inductor 220 shown as part of the DC to DC converter can be effectively replaced in favour of the PM machine stator windings. U.S. Pat. No. 7,595,612 provides an example of a drive similar to that shown in FIG. 2.
A drawback with the simple arrangement shown in FIG. 2 is that the use of a 6 pulse rectifier introduces significant levels of 5th and 7th harmonic currents into the system and stator windings. These currents are parasitic in that they do not contribute to the useful electrical power generated and create additional heating in the stator windings, thereby reducing the current carrying capacity. Further, the harmonic currents cause eddy current heating in the permanent magnets of the rotor and create additional torque oscillations in the mechanical drive train.
It is known that providing a 12-pulse system can help reduce the 5th and 7th harmonic currents in the PM machine. A two channel approach is known to construct 12-pulse transformer-rectifier system in high power electrical power transmission and distribution systems, and a similar approach may be applicable to a PM machine. Another known configuration is a 6 pulse synchronous system achieved by connecting the stator windings in a star and delta configuration which provides two channels electrically displaced 30 degrees.
The arrangement 310 shown in FIG. 3a includes a star-delta (YD) stator winding 314 arrangement to provide the two channels 316a,b which are 30 degrees out of phase with one another. The two channels 316a,b are connected to respective 6 pulse diode rectifiers 318a,b which each feed respective loads 320a,b. The 5th and 7th harmonic currents can be calculated and analysed after an FFT analysis and also measuring the Total Harmonic Distortion (THD) As can be seen from FIG. 3b, the 30 degree phase shift provided by the YD windings 314 results in the 5th and 7th harmonic currents being out of phase so as to largely cancel out one another within the stator. This reduces rotor eddy current heating and the torque ripple seen by the rotor.
Although this provides an improvement over the PM machine with conventional stator windings, the YD stator windings 314 are overly complex and do not address the additional current flow in the windings which still suffer from parasitic power dissipation as a result of the 5th and 7th harmonics.
The present invention seeks to provide an improved PM machine for DC systems.